Y and Nb-doped SrTiO3 as a mixed conducting anode for solid oxide fuel cells

ABSTRACT

The invention disclosed relates to novel materials of the general formula
 
(Sr 1-1.5x   M   1   x ) 1-y/2 Ti 1-y   M   2   y O 3   [I]
 
wherein M 1  is a first trivalent dopant metal atom replacing some of the strontium atoms on a strontium sub-lattice, x is a mole percent of said dopant atoms M 1  on the strontium sublattice and 0&lt;x≦0.04, and M 2  is a second pentavalent dopant metal atom replacing some titanium atoms on a titanium sublattice, y is a mole percent of said dopant atoms M 2  on the titanium sublattice and 0&lt;y≦0.2. Also disclosed is a novel reduced form of the compounds of formula I, ie. compounds of formula (Sr 1-1.5x M 1   x ) 1-y/2 Ti 1-y M 2   y O 3-δ  II. The variability in oxygen content between the oxidized and reduced forms of these compounds corresponds to 0&lt;δ≦0.7. These novel compounds maintained a stable single phase at both high and low oxygen partial pressures. Also disclosed is a solid oxide fuel cell including an anode made of the novel compounds of formula I.

This application claims the benefit of U.S. application Ser. No.60/482,714, filed Jun. 27, 2003.

BACKGROUND OF THE INVENTION

This invention relates to SrTiO₃ co-doped with a first trivalent metaldopant atom (e.g. yttrium) replacing some of the Sr atoms on a strontiumsub-lattice in the A site of the molecule and a second pentavalentdopant metal atom(e.g. niobium) replacing some of the Ti atoms on atitanium sub-lattice in the B site of the molecule, and to solid oxidefuel cells including an anode made of the co-doped SrTiO₃.

The solid oxide fuel cell (SOFC) is one of the most advanced systems forgenerating electricity in an efficient and environmentally friendly way.It can operate on a variety of fuels in addition to hydrogen, and mayfind applications from transportation to stationary systems (1, 2).

There has been a long history of research on the application of cermets(ceramic-metal composites) of nickel and yttria stabilized zirconia(YSZ) as anodes in solid oxide fuel cells. However, Ni—YSZ has a numberof drawbacks including sintering of the Ni particles at high operatingtemperatures, sulfur poisoning and carbon deposition when the SOFC isfuelled by natural gas, reliance on a triple phase junction for theelectrochemical reaction and cost. Ni—YSZ cermet anodes cannot be usedin pure methane without pre-reforming. Therefore, an alternativematerial with adequate conductivity (>50-100 S/cm at 800-1000° C.) andcatalytic activity toward total or partial oxidation of methane isnecessary. Mixed ionic-electronic conductor (MIEC) oxides, eitherperovskite type (3-5) or non-perovskite (6-9) type, exhibiting highionic and electronic conductivities at elevated temperatures areattractive candidates for SOFC electrodes. Mixed conductors are believedto relax the limitation attributed to electronic conducting electrodesby expanding the active reaction zone to include the whole of theelectrode-gas interface instead of so-called triple phase boundary area(10).

SUMMARY OF THE INVENTION

According to one embodiment of the invention, a novel compound ofgeneral formula I is provided(Sr_(1-1.5x) M1_(x))_(1-y/2)Ti_(1-y) M2_(y)O₃  Iwherein M1 is first trivalent dopant metal atom replacing some strontiumatoms on a strontium sublattice, x is a mole percent of said dopantatoms M1 on the strontium sublattice and 0<x≦0.04, and M2 is secondpentavalent dopant metal atom replacing some titanium atoms on atitanium sublattice, y is a mole percent of said dopant atoms M2 on thetitanium sublattice and 0≦y≦0.2.

According to an aspect of this embodiment of the invention, M1 isyttrium (Y⁺³) and wherein x is in a range from 0 to 4 mole % and M2 isniobium (Nb⁺⁵) and wherein y is in a range from 0 to 20 mole %.

According to another aspect of this embodiment of the invention,0<x≦0.04 and 0≦y≦0.2.

According to yet another aspect of this embodiment of the inventionx=0.04 and 0≦y≦0.2.

According to yet another aspect of this embodiment of the invention,0.03≦x≦0.04 and 0.05≦y≦0.20.

According to another embodiment of the invention, a novel compound ofgeneral formula II is provided(Sr_(1-1.5x) M1_(x))_(1-y/2)Ti_(1-y) M2_(y)O_(3-δ)  II,wherein M1 is first trivalent dopant metal atom replacing some strontiumatoms on a strontium sublattice, x is a mole percent of said dopantatoms M1 on the strontium sublattice and 0<x≦0.04, and M2 is secondpentavalent dopant metal atom replacing some titanium atoms on atitanium sublattice, y is a mole percent of said dopant atoms M2 on thetitanium sublattice and 0≦y≦0.2 and δ is in a range from 0 to about0.07.

According to an aspect of this embodiment of the invention, M1 isyttrium (Y⁺³) and wherein x is in a range from 0 to 4 mole % and M2 isniobium (Nb⁺⁵) and wherein y is in a range from 0 to 20 mole %.

According to another aspect of this embodiment of the invention,0<x≦0.04and 0≦y≦0.2.

According to yet another aspect of this embodiment of the inventionx=0.04 and 0≦y≦0.2.

According to yet another aspect of this embodiment of the invention,0.03≦x≦0.04 and 0.05≦y≦0.20.

According to another embodiment of the invention, a process is providedfor making a compound of formula I.

That is, the compounds of formula I were prepared in air in which theA-site deficiency is introduced to balance the charge difference ofpartially substituting Y³⁺ for Sr²⁺ and Nb⁵⁺ for Ti⁴⁺. Thestoichiometric ratios of A site elements (Sr²⁺, Y³⁺) and B site elements(Ti⁴⁺, Nb⁵⁺) were adjusted to match the electrically neutral conditionunder oxidation atmosphere.

The compounds of formula I with all species in their fully oxidizedstates are then partially reduced at elevated temperature in a forminggas e.g. Ar-8% H₂, to obtain the novel oxygen deficient compounds offormula 11.

Therefore, “δ”(delta) will be “0” under air. However, the coupling ofTi⁴⁺/Nb⁵⁺ can be reduced to Ti³⁺/Nb⁴⁺ under reducing atmosphere at hightemperature. Empirically we calculated the δ (delta) of the formula IIbased on thermo-gravimetric data as shown in FIG. 6. The range of δ(delta) depends on the range of y in the formula and can be defined as0≦δ≦0.07 in case of 0≦y≦0.2 for x=0.04. In this case, both electronicand ionic conduction is achieved which is a favorable criterion for SOFCanode performance. The thermal and electrical behavior of these novelceramics was measured, and is described below.

According to another embodiment of the invention, a solid oxide fuelcell is provided, comprising a laminate of a cathode, an anode and anelectrolyte sandwiched between said cathode and anode, said anodecomprising a compound of general formula (Sr_(1-1.5x)M1_(x))_(1-y/2)Ti_(1-y)M2 _(y)O₃ I, wherein M1 is first trivalent dopantmetal atom replacing some strontium atoms on a strontium sub-lattice, xis a mole percent of said dopant atoms M1 on the strontium sub-latticeand 0<x≦0.04, and M2 is second pentavalent dopant metal atom replacingsome titanium atoms on a titanium sub-lattice, y is a mole percent ofsaid dopant atoms M2 on the titanium sub-lattice and 0≦y≦0.2.

According to an aspect of this embodiment of the invention, M1 isyttrium (Y³⁺) and wherein x is in a range from 0 to 4 mole % and M2 isniobium (Nb⁵⁺) and wherein y is in a range from 0 to 20 mole %.

According to another aspect of this embodiment of the invention,0.03≦x≦0.04 and 0.05≦y≦0.20.

According to another aspect of this embodiment of the invention, x=0.04and y=0.20.

According to another aspect of this embodiment of the invention, theelectrolyte is yttrium stabilized zirconia (YSZ) and the anode haschemical stability therewith at operating temperature and also at theSOFC sintering temperature of up to 1350° C. It will be appreciated bythose skilled in the art that the electrolyte and cathode may be made ofany materials conventionally used for such components in commercial SOFCfuel cells. For examples of such materials see refs. 13 and 14, thedisclosures of which are incorporated herein by reference.

According to another aspect of this embodiment of the invention, theanode includes a metallic catalyst and/or a ceramic catalyst mixed withsaid compound of formula I, for increasing conductivity and/or catalyticactivity of said anode.

According to another aspect of this embodiment of the invention, themetallic catalyst is selected from the group consisting of iron, cobalt,nickel, copper and alloys thereof.

According to another aspect of this embodiment of the invention, theceramic catalyst is selected from the group consisting of undoped ceriumoxide, doped cerium oxide, lanthanum and chromium based perovskites.

According to another aspect of this embodiment of the invention, thesolid oxide fuel cell is operated at a temperature ranging from 400° C.to 1000° C., and wherein an anode compartment is operated at an oxygenpressure ranging from about 10⁻¹⁴ to about 10⁻²² atm.

In another embodiment of the invention, an electrolyte-supported singlecell SOFC with (Sr_(0.94)Y_(0.04))_(0.9)Ti_(0.8)Nb_(0.2)O_(2.93) as theanode was made and tested at 900° C. using forming gas (Ar-8% H₂) as afuel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is lattice parameters of yttrium doped strontium titanate as afunction of the concentration of yttrium.

FIG. 2 is a graph illustrating XRD patterns of(Sr_(0.94)Y_(0.04))_(1-y/2)Ti_(1-y)Nb_(y)O₃ sintered in air at 1400° C.for 4 h.

FIG. 3 is a graph illustrating variation of the lattice parameters of(Sr_(0.94)Y_(0.04))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ) sintered in air andforming gas.

FIG. 4 is a graph illustrating the electrical conductivity of(Sr_(0.94)Y_(0.4))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ) at 800° C. as a functionof oxygen partial pressure.

FIG. 5 is a graph illustrating the conductivity of(Sr_(0.94)Y_(0.04))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ) versus temperature informing gas.

FIG. 6 is a graph of oxygen deficiency versus doping level of niobium in(Sr_(0.94)Y_(0.04))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ).

DETAILED DESCRIPTION OF THE INVENTION

Compounds of formula (Sr_(1-1.5x)Y_(x))_(1-y/2)Ti_(1-y)Nb_(y)O₃ wherein0<x≦0.04 and 0≦y≦0.2 were prepared by conventional solid-state reaction.High purity SrCO₃ (Aldrich, 99.9%), Y₂O₃ (Aldrich, 99.99%), TiO₂ (AlfaAesar, 99.8%, rutile), and Nb₂O₅ (Aldrich, 99.9%) were intimately mixedin the appropriate ratios, and heated, in air, at 1000° C. for 5 h, thenat 1200° C. for 10 h, and finally, reground and reheated at 1400° C. for10 h. Dense pellets (17 mm diameter) and bars (ca. 5×2.5×20 mm) of eachsample were obtained by pressing at 1000 kg/cm² and then sintering at1400° C. for 4 h in forming gas (Ar-8% H₂). Polyvinyl alcohol (PVA) andpolyethylene glycol (PEG) were used as the binder and the plasticizer,respectively. The compounds were sintered on yttria stabilized zirconia(YSZ) substrates. No reaction with the substrate was detected aftersintering.

Diffraction patterns were recorded on powders and ceramics on a BrukerD8 Advanced X-ray diffractometer (Bruker AXS Inc.) with Cu K_(α)radiation. For accurate lattice parameter determination, silicon waschosen as a standard. Complete patterns were recorded over the range of2θ from 20 to 70°. The unit cell parameters were derived from acomputerized least-squares refinement technique.

The conductivity measurements were made using a conventional four-probetechnique. Pt mesh was attached to the surface with Pt paste 4082 (FerroInc.). The contacts were cured in situ under forming gas for 1 hour at950° C. The temperature was then reduced to 800° C. to perform theconductivity measurements. Initially, argon with trace amounts of oxygenwas allowed to leak into the measurement cell over a period of 5-6 hoursand the conductivity monitored with changing oxygen partial pressuremeasured by a POAS micro-sensor (Setnag). The conductivity as a functionof temperature was also recorded.

To determine the oxygen content in the reduced compounds,thermogravimetric analysis was performed using a Hi-Res TGAthermogravimetric analyzer (TA Instruments). The samples were oxidizedby heating at 5° C./min up to 1000° C. in flowing air.

Thermal expansion coefficients were measured in the temperature range of100-800° C. using TMA 2940 thermomechanical analyzer (TA Instruments).

The lattice parameters of perovskite yttrium doped strontium titanate asa function of yttrium contents are shown in FIG. 1. It can be seen thatsingle-phase cubic perovskite-type solid solutions of yttrium dopedstrontium titanate were observed for compositions containing up to 4 mol% of yttrium. The effect of yttrium doping into perovskite strontiumtitanate on enhancing mixed conductivity under reducing atmosphere waswell described in literature (3). The conductivity was found to increasewith increasing concentration of yttrium. Therefore compounds of Y andNb co-doped strontium titanate with 4 mol % doping of yttrium wereinvestigated to determine the effect of the solid solubility of niobiumon the titanium sublattice. Single-phase compounds were observed bypowder X-ray diffraction for y<0.2. For y=0.2 nearly single-phasesamples were observed with a very small extra peak at 2θ=30.5° (see FIG.2). This peak can be assigned to Y₂Ti₂O₇. Therefore, niobium probablydecreases the solubility level of yttrium in SrTiO₃. Any furtherincrease of Nb in the system results in an increase of second phasecontent. Cell parameters for the(Sr_(0.94)Y_(0.004))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ) compounds sintered inair and forming gas at 1400° C. are given in Table 1. As the Nb contentincreases, the unit cell expands (see FIG. 3), due to the larger radiusof Nb⁵⁺ (0.64 Å) versus Ti⁴⁺ (0.605 Å) (11). The diffraction patternsresemble very closely that of cubic SrTiO₃. Lattice parameters weredetermined by indexing the diffraction peaks by analogy with those ofSrTiO₃. On reduction all samples exhibited an approximate 0.03-0.15%expansion in crystallographic unit cell size due to the larger sizes ofTi³⁺ (0.67 Å) and Nb⁴⁺ (0.68 Å).

TABLE I Crystallographic unit cell parameters and densities calculatedfrom the crystallographic data for (Sr_(0.94)Y_(0.04))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ) sintered in air and forming gas at 1400° C. for 4hours. Calculated density,/ a₀, Å Expansion, cm³ Y Air FG % Air FG 03.9007 3.9020 0.033 5.085 5.073 0.05 3.9064 3.9085 0.054 5.066 5.0440.10 3.9093 3.9141 0.122 5.057 5.019 0.20 3.9160 .9217 0.145 5.037 4.987

Four probe dc conductivity measurements of the compounds measured at 800° C. over a wide range of oxygen partial pressures are shown in FIG. 4.The conductivity of all Nb-doped titanates increases slightly withdecreasing oxygen partial pressures, indicating n-type conductivitypredominated under reducing conditions. The electron concentration, orTi³⁺/Nb⁴⁺ concentration is saturated by the Nb doping level, and theconductivities were only a weak function of oxygen partial pressure inthe range 10⁻²²-10⁻¹⁵ atm. One may see that the Y and Nb co-dopedcompound with x=0.04 and y=0.2 has much higher conductivity thanSr_(0.94)Y_(0.04)TiO₃ at 800° C. in the whole range of oxygen partialpressures measured. It would be desirable to use this material as aporous anode on which a thin layer of electrolyte is attached.Electrical conductivity of(Sr_(1-1.5x)Y_(x))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ) may be influenced byfactors such as excess strontium/yttrium vacancies and the existence ofTi³⁺ (or Nb⁴⁺) which are produced under reducing conditions. Bulkdensities and conductivities of the various(Sr_(0.94)Y_(0.04))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ) compositions are givenin Table 2.

TABLE 2 Density, oxygen deficiency and conductivity of(Sr_(0.94)Y_(0.04))_(1-y/2) Ti_(1-y)Nb_(y)O_(3-δ) sintered at 1400° C.in forming gas for various levels of Nb doping. Bulk density, Relativedensity, ^(*)Conductivity, Y g/cm³ % δ S/cm 0 3.99 78.7 0.017 18.4 0.054.04 80.1 0.032 30.0 0.10 4.06 78.2 0.044 43.8 0.20 4.13 82.8 0.064 62.8^(*)Measured at 800° C. in forming gas.

For the proposed application as an anode in a SOFC, it is important toinvestigate the variation of conductivity of(Sr0.94Y0.04)_(1-y/2)Ti_(y)Nb_(y)O_(3-δ) in forming gas as a function oftemperature. FIG. 5 shows the conductivity measured from low to hightemperature. It is well known that the oxygen partial pressure informing gas is determined by the equilibrium:H₂ (g)+½O₂ (g)←→H₂O (g)

and significantly decreases with decreasing temperature. Therefore, theincrease of conductivity with decreasing temperature is very likelyowing to the decrease of oxygen partial pressure. However, when thetemperature approaches a kinetic limit (in this case 250-350° C.) thesystem may not be in thermodynamic equilibrium and the temperaturemainly controls the generation of electrons rather than the reduction ofoxygen partial pressure.

Thermogravimetric measurements were performed on(Sr_(0.94)Y_(0.04))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ) compounds as preparedunder reducing conditions. The compounds were heated under flowing airup to 1000° C. to complete oxidation. Measurements showed that oxidationof the compounds begins at 840-870° C. The weight gain associated withthe oxidation process was used to determine the amount of oxygen performula unit. The calculated oxygen deficiency as a function of Nbcontent is plotted in FIG. 6. The δ monotonically increases withincreasing Nb concentration.

The thermal expansion coefficients of(Sr0.94Y_(0.04))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ) compounds were comparedwith that of (Y₂O₃)_(0.08)(ZrO₂)_(0.92). All materials showed nearlinear expansion, with similar thermal expansion coefficients. Theobserved value of α=11.6×10⁻⁶ K⁻¹ (from 100° C. to 800° C.) for y=0.2was close to the 10.8×10⁻⁶K⁻¹ reported for YSZ (12). The measuredthermal expansion coefficients of the other Nb-doped (y<0.2) ceramicswere a little higher, in the range 11.6-12.7×10⁻⁶K⁻¹.

The anode material must have chemical compatibility with theelectrolyte, not only at operating temperature but also at the highertemperatures that the anode is exposed to during the fabrication of theSOFC. Chemical compatibility was tested by grinding a 1:1 (by weight)mixture of TZ-8Y (Tosoh Zirconia) and(Sr_(0.94)Y_(0.04))_(0.9)Ti_(0.8)Nb_(0.2)O₃ and firing at 1350° C. for10 h in forming gas. No changes in the X-ray patterns were observedafter this heat treatment.

(Sr_(0.94)Y_(0.04))_(1-y/2)Ti_(1-y)Nb_(y)O_(3-δ) compounds have suitablefeatures for application as an anode material for a SOFC in the sensethat they are stable in both air and reducing conditions, exhibit quitehigh (62.8 S/cm at y=0.2) conductivity in low Po₂, and do not show anyevidence of chemical reaction with YSZ electrolyte even at 1350° C.

A single cell SOFC was made by screen-printing(Sr_(0.94)Y_(0.04))_(0.9)Ti_(0.8)Nb_(0.2)O₃-glycol slurry on one side ofa YSZ disc (18 mm diameter, 0.35 mm thick) and firing at 1200° C. informing gas. Pt paste (Ferro Inc.) was applied on other side of the diskas the cathode. Forming gas saturated with water vapor at 20° C. wasdelivered to the anode and air was provided to the cathode.Unfortunately the cell showed poor power density (max 15 mW/cm² at 900°C.). Specific electrode interfacial resistance was roughly estimated at700 Ω·cm². This indicates that the main problem may lie with theinterface contacts, rather than with the bulk properties of the anode.

REFERENCES

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1. A compound having a formula I;(Sr_(1-1.5x) M1_(x))_(1-y/2)Ti_(1-y) M2_(y)O₃  [I] wherein M1 is ayttrium (Y³⁺) dopant metal atom replacing some strontium atoms on astrontium sublattice, x is a mole percent of said dopant atoms M1 on thestrontium sublattice and 0<x≦0.04, and M2 is a niobium (Nb⁵⁺) dopantmetal atom replacing some titanium atoms on a titanium sublattice, y isa mole percent of said dopant atoms M2 on the titanium sublattice and0<y≦0.2.
 2. The compound according to claim 1, wherein x is in a rangefrom 0.02 to 0.04 and wherein y is in a range from 0.05 to 0.20.
 3. Thecompound according to claim 1, wherein 0.03≦x≦0.04 and 0.05≦y≦0.20. 4.The compound according to claim 3, wherein x=0.04 and y=0.20.
 5. Thecompound according to claim 1, wherein said compound has a mixedelectronic and ionic conductivity greater than 5 S/cm at a temperatureranging from 400° C. to 1000° C. and an oxygen pressure ranging fromabout 10⁻¹⁴ to about 10⁻²² atm.
 6. The compound according to claim 1,wherein said compound has a coefficient of thermal expansion (CTE)ranging from 11×10⁻⁶K⁻¹ to 13×10⁻⁶K⁻¹ under reducing atmosphere.
 7. Acompound having a formula II(Sr_(1-1.5x) M1_(x))_(1-y/2)Ti_(1-y) M2_(y)O_(3-δ)  II wherein M1 is ayttrium (Y³⁺) dopant metal atom replacing some strontium atoms on astrontium sublattice, x is a mole percent of said dopant atoms M1 on thestrontium sublattice and 0<x≦0.04, and M2 is a niobium (Nb⁵⁺) dopantmetal atom replacing some titanium atoms on a titanium sublattice, y isa mole percent of said dopant atoms M2 on the titanium sublattice and0<y≦0.2 and 0<δ≦0.7.
 8. The compound according to claim 7, wherein x isin a range from 0.02 to 0.04 and wherein y is in a range from 0.05 to0.20.
 9. The compound according to claim 7, wherein 0.03≦x≦0.04 and0.05≦y≦0.20.
 10. A solid oxide fuel cell, comprising; a laminate of acathode, an anode and an electrolyte sandwiched between said cathode andanode, said anode comprising a compound of general formula(Sr_(1-1.5x)M1 _(x))_(1-y/2)Ti_(1-y)M2 _(y)O₃ I, wherein M1 is a yttrium(Y³⁺) dopant metal atom replacing some strontium atoms on a strontiumsublattice, x is a mole percent of said dopant atoms M1 on the strontiumsublattice and 0<x≦0.04, and M2 is a niobium (Nb⁵⁺) dopant metal atomreplacing some titanium atoms on a titanium sublattice, y is a molepercent of said dopant atoms M2 on the titanium sublattice and0<y≦y≦0.2.
 11. The solid oxide fuel cell according to claim 10, whereinx is in a range from 0.02 to 0.04 and wherein y is in a range from 0.05to 0.20.
 12. The solid oxide fuel cell according to claim 10, wherein0.03≦x≦0.04 and 0.05≦y≦0.20.
 13. The solid oxide fuel cell according toclaim 10, wherein x=0.04 and y=0.20.
 14. The solid oxide fuel cellaccording to claim 10, wherein the electrolyte is yttrium stabilizedzirconia (YSZ) and said anode has chemical stability therewith atoperating temperature and also at the SOFC sintering temperature of upto 1350° C.
 15. The solid oxide fuel cell according to claim 10, whereinsaid anode includes a metallic catalyst and/or a ceramic catalyst mixedwith said compound of formula I, for increasing conductivity and/orcatalytic activity of said anode.
 16. The solid oxide fuel cellaccording to claim 10, wherein said metallic catalyst is selected fromthe group consisting of iron, cobalt, nickel, copper and alloys thereof.17. The solid oxide fuel cell according to claim 15, wherein saidceramic catalyst is selected from the group consisting of undoped ceriumoxide, doped cerium oxide, doped lanthanum and chromium basedperovskites.
 18. The solid oxide fuel cell according to claim 10,operating at a temperature ranging from 400° C. to 1000° C., and whereinan anode compartment is operated at an oxygen pressure ranging fromabout 10⁻¹⁴ to about 10⁻²² atm.
 19. A process for producing a compoundof formula I(Sr_(1-1.5x) M1_(x))_(1-y/2)Ti_(1-y) M2_(y)O₃  [I] wherein M1 is ayttrium (Y³⁺) dopant metal atom replacing some strontium atoms on astrontium sublattice, x is a mole percent of said dopant atoms M1 on thestrontium sublattice and 0<x≦0.04, and M2 is a niobium (Nb³⁺) dopantmetal atom replacing some titanium atoms on a titanium sublattice, y isa mole percent of said dopant atoms M2 on the titanium sublattice and0<y≦0.2, comprising, replacing some strontium atoms on a strontiumsublattice of a compound of formula SrTiO₃ by a yttrium (Y³⁺) dopantmetal atom M1 and replacing some titanium atoms on a titaniumsub-lattice by a niobium (Nb⁵⁺) dopant metal atom M2, to balance thecharge difference of partially substituting Y³⁺ for Sr²⁺ and Nb⁵⁺ forTi⁴⁺, and adjusting the stoichiometric ratios of A site elements (Sr²⁺,Y³⁺) and B site elements (Ti⁴⁺, Nb⁵⁺) to match the electrically neutralcondition under oxidation atmosphere.
 20. A process for producing acompound of formula II,(Sr_(1-1.5x) M1_(x))_(1-y/2)Ti_(1-y) M2_(y)O_(3-δ)  II wherein M1 is ayttrium (Y³⁺) dopant metal atom replacing some strontium atoms on astrontium sublattice, x is a mole percent of said dopant atoms M1 on thestrontium sublattice and 0<x≦0.04, and M2 is a niobium (Nb⁵⁺) dopantmetal atom replacing some titanium atoms on a titanium sublattice, y isa mole percent of said dopant atoms M2 on the titanium sublattice and0<y≦0.2 and to 0<δ≦0.7 comprising partially reducing a compound offormula I as defined in claim 19 at elevated temperature in a forminggas.